The present application relates to a phosphor, and particularly to a phosphor that includes (Sr1−x, Bax)3SiO5 as a base material and europium (Eu) as an activator and emits orange fluorescence, a method for manufacturing the phosphor, and a light-emitting device and a display device using the phosphor.
In recent years, blue light-emitting diodes (LEDs) mainly composed of gallium nitride that can efficiently emit light in a blue light wavelength range have been developed and are widely used. A white light-emitting device (white LED) has been developed by combining such a blue LED with a yellow phosphor, which emits yellow fluorescence under excitation with light in a blue-light wavelength range radiated from the blue LED. Since the white LED has a spectrum covering a wide wavelength range, its brightness in consideration of a visibility curve is high. Therefore, the white LED is used for an optical device such as a display device attached to a cellular phone or a camcorder. It is also used as a backlight of liquid crystal displays as a substitute for an existing small lamp or fluorescent lamp or the like. For example, (Y, Gd)3(Al, Ga)5O12:Ce is used as a yellow phosphor.
The white LED achieved by combining a blue LED with a yellow phosphor has a problem in that color rendering in a red-color region is low due to lack of red-light emission. Accordingly, a red phosphor that emits red fluorescence and an orange phosphor that emits orange fluorescence when excited with light in a blue-light wavelength range are actively developed. However, a red phosphor with an emission wavelength of 600 nm or more necessarily has a high proportion of covalent bonds in its crystal structure. Thus, only a few crystals are available, and sulfides and nitrides are the crystals most reported as a red phosphor.
There are some reports about an orange phosphor composed of a silicate of an alkaline-earth metal.
Japanese Unexamined Patent Application Publication No. 2005-68269 (paragraphs 0010 to 0014) (Patent Document 1) titled “PHOSPHOR AND TEMPERATURE SENSOR USING THE SAME” provides the following descriptions.
A base material constituting a phosphor according to the application of Patent Document 1 is a silicate of an alkaline-earth metal and is represented by a general formula Mx(SiOn)y, where n is a whole number of 3 or more, preferably 3 or more and 5 or less. In other words, silicon trioxide (SiO3), silicon tetroxide (SiO4), and silicon pentoxide (SiO5) are preferred as the silicate. This is presumably because they can form a suitable crystal structure as a base material of a phosphor.
In the general formula, M represents one or more kinds of alkaline-earth metal elements. Examples of the alkaline-earth metals include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Among these alkaline-earth metals, Sr and Ba are preferred because they can form a suitable crystal structure as a base material of a phosphor. An alkaline-earth metal M can be particularly represented by (Ba1−a, Sra), where a is a mixed crystal ratio (0≦a≦1). In the general formula, x and y each represents a whole number of 1 or more and x is determined in accordance with n and y. In the case of y=1 in the general formula, x≧1 is preferred. This means that, comparing an alkaline-earth metal with a silicate both contained in a base material on a molar basis, the number of moles of the alkaline-earth metal is larger than or equal to that of the silicate. This is presumably because such a composition is necessary to form a suitable crystal structure as a base material of a phosphor.
Examples of the silicate of the alkaline-earth metal that satisfies the conditions described above include Ba3SiO5, Sr3SiO5, (Ba1−a, Sra)3SiO5, Ba2SiO4, and α-BaSiO3. Among these, Ba3SiO5, Sr3SiO5, and (Ba1−a, Sra)3SiO5 have a tetragonal Cs3CoCl5-type crystal structure.
The phosphor includes a silicate of the alkaline-earth metal as a base material and lanthanoid ions as an activator, and is represented by a general formula, La:Mx(SiOn)y, where La is a lanthanoid. The lanthanoid is one of europium (Eu) and cerium (Ce), and is contained in the phosphor by taking form of Eu2+ if it is Eu and Ce3+ if it is Ce, for example. Eu or Ce is added to a base material as an oxide by taking form of Eu2O3 if it is Eu and CeO2 if it is Ce. In this case, 0.001 to 0.2 atomic percent of the lanthanoid (La) relative to 1 atom of the alkaline-earth metal (M) is preferably added. In the case of less than 0.001 atomic percent, emission intensity decreases and sufficient brightness is not achieved. In the case of more than 0.2 atomic percent, quenching of light called concentration quenching is likely to occur.
Examples of the phosphor include Eu2+:Ba3SiO5, Ce3+:Ba3SiO5, Eu2+:Sr3SiO5, Ce3+:Sr3SiO5, Eu2+:(Ba1−a, Sra)3SiO5, Ce3+:(Ba1−a, Sra)3SiO5, Eu2+:Ba2SiO4, Ce3+:Ba2SiO4, Eu2+:BaSiO3, and Ce3+:BaSiO3.
In Japanese Unexamined Patent Application Publication No. 2006-36943 (paragraphs 0012 to 0014 and 0019 to 0020) (Patent Document 2) and Japanese Unexamined Patent Application Publication No. 2007-227928 (paragraphs 0017 and 0027 and FIG. 4) (Patent Document 3), a Sr3SiO5 phosphor is described, and Patent Document 2 titled “ORANGE PHOSPHOR” provides the following descriptions.
An orange phosphor according to the application of Patent Document 2 has a single phase crystal structure represented by the general formula (Sr1−x, EUx)3SiO5 (0<x≦0.10) and exhibits a high-intensity orange light emission with a peak at around 580 nm.
At a firing temperature of 1300° C. or more or when firing is conducted at more than 1300° C., a phosphor having a single phase crystal structure that matches with a powder X-ray diffraction pattern of Sr3SiO5 registered in Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 26-984 is achieved. This phosphor exhibits a high-intensity orange light emission with a peak at around 580 nm.
The orange phosphor has a crystal structure of Sr3SiO5 and a structure in which part of Sr is replaced with Eu as an activator. The ratio in which Sr is replaced with Eu is more than 0% but not more than 10% of the atomic weight of Sr. This is because light emission does not occur if Sr is not replaced with Eu. Furthermore, if more than 10% of Sr is replaced with Eu, the phosphor does not exhibit a high-intensity light emission due to photoexcitation in an ultraviolet to visible range caused by concentration quenching, formation of a multiphase crystal structure, or the like.
In Non-Patent Document 1 (J. K. Park et al, “Embodiment of the warm white-light-emitting diodes by using a Ba2− codoped Sr3SiO5:Eu phosphor”, Appl. Phys. Lett., 88, 043511-1 to -3 (2006) (FIG. 1 and line 12 in right column of 043511-1 through line 15 in right column of 043511-2)), fluorescence spectra of Sr3SiO5:Eu samples ((Sr2.93−x, Bax)SiO5:Eu0.07 (x=0.0, 0.05, 0.10, and 0.20) as a function of Ba2+ concentration are described. In the description, the entire spectra shift to longer wavelengths as the Ba2− concentration increases, and the peak wavelength shifts from 570 to 585 nm. In addition, the second phase, BaSi4O9 is formed when the Ba2+ concentration exceeds 0.5 mol.
Non-Patent Document 2 (H. S. Jang, W. B. Im and D. Y. Jeon: “Luminescent properties of (Sr1−xMx)3SiO5:Eu2+ (M=Ca, Ba) phosphor for white emitting light source using blue/near UV LEDs” Proc. IDW/AD'05 (2005) pp. 539-pp. 542) (FIG. 3, FIG. 4, Abstract, Results and discussion, Conclusion)) provides the following descriptions. When the number of Eu2+ ions in a Sr3SiO5 host lattice increases, a shift of the emission wavelength to longer wavelengths is actually observed. In photoluminescence (PL) spectra of (Sr1−x, Bax)3SiO5:Eu2+ having a Ba concentration of 0 mol %, 20 mol %, 40 mol %, 60 mol %, and 80 mol %, when Sr sites are replaced with 20 mol % of Ba2+, the emission band shifts to longer wavelengths, that is, a red shift occurs. When the Ba concentration exceeds 20 mol %, the emission band shifts to shorter wavelengths.